While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail, a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiment illustrated.
As industrial automation advances, interconnectivity between various microprocessor based plant floor devices, such as programmable logic controllers ("PLCs") and put computers, becomes more and more desirable. Various schemes have been developed to interconnect PLCs and host computers, but their applications have been limited.
For example, if one wanted to communicatively couple three PLCs in the absence of a network, each PLC would typically require a separate serial, or point to point, connection with the other two PLCs. The speed of serial communication is limited. Further, as the number of interconnected PLCs grows linearly, the number of serial connections grows geometrically.
In one of the above-incorporate co-pending patent application Ser. No. 258,779 now U.S. Pat. No. 4,992,926, peer-system is disclosed for interconnecting a plurality of PLCs. However this system requires a dedicated network.
Allen-Bradley Company, Inc., in conjuction with Digital Equipment Corportion ("DEC") has developed a system marketed under the trade name "Pyramid Integrator" for interconnecting devices over the relatively standardized Ethernet network via DEC's VAX computer. However according to this system, only up to four PLCs can be coupled to an Ethernet network per VAX computer, and each of the PLCs must be plugged into the backplane of the VAX computer. If five PLCs are needed on the Ethernet, two VAX computers are required. This can greatly add to the expense of automation.
In addition as is well known, host computers concurrently perform a plurality of applications programs, or tasks. When a PLC is connected to such a host computer, it is often important for the host computer to obtain data from the PLC. Typically this is accomplished by having the host computer poll the PLCs. However, this polling either requires the host computer to interrupt the PLC's processing of its ladder program or it requires the host computer to wait for the PLC to complete a scan of its ladder program. Further it is often important for the PLC to send unsolicited information to the host computer.
Data typically is transmitted between microprocessor based devices on an Ethernet network in the form of data packets. The packets generally include a preamble portion comprising routing information and protocol type, a user defined portion comprising the message itself, and an error detection portion. As the speed of communication between microprocessor based devices increases, error detection becomes more and more critical. Typically the error detection views the entire data packet to determine existence of an error. This often does not detect errors in the user data portion alone. Further, the protocol often cannot accurately respond to lost messages.
Finally as automated systems control ever larger operations, handling and prioritizing of event notifications or alarms, such as faults, alerts and warning, by the host computers becomes more important. While certain host computers have been able to receive alarms, they have been received on a global basis, rather than individually on a task basis.
Applicants' invention is provided to solve these and other problems.